Steam engines and other external combustion engines have been known for years. They have been used on a variety of vehicles and equipment to perform work. For example, they have been used in steamboats, steam locomotives, as electrical generators and even in some of the very first automobiles. External combustion engines use a fuel source, such as wood or coal, to generate heat. Instead of burning the fuel to directly generate power, this heat is used to heat a liquid fluid such as water to its boiling point. Once the water becomes vapor, additional heat allows the pressure in a boiler to increase. It is this pressure that is needed to generate power to the engine.
Once the pressure in the boiler has reached the desired pressure point, the pressure causes portions of the engine to move. For example in a piston driven engine, the pressure that is built up in the boiler causes the pistons to move. The movement of the pistons transfers the power from the steam to the engine, and thus to a shaft or other rotating device. The steam in the cylinder cools as the piston expands in the cylinder. This cooled steam is either exhausted by the engine into the atmosphere or recovered for later use by the steam engine.
There are two problems commonly associated with steam engines that make their use in vehicles, especially on-demand vehicles such as personal automobiles, undesirable. First, steam engines typically require a significant amount of time to warm up and produce motive power. This could take upwards of 5-10 minutes to generate enough steam to move the vehicle at highway speeds. While this amount of time to warm up the boiler is sometimes acceptable in larger/scheduled vehicles, such as trains and boats, it is generally not acceptable in automobiles. Second, typical steam engines require a large storage area for storing the steam as it is generated, prior to injecting the steam into the engine. This large storage area takes up a considerable amount of space in a vehicle that would desirably be available for cargo or passengers.